Natural Selection on Polygenic Traits

Mechanisms of Evolution The main causes of evolutionary change are genetic drift, gene flow, and natural selection. Natural selection is the most important, because it is the only process that promotes adaptation.

Mechanisms of Evolution: Genetic Drift Allele can become more or less common BY CHANCE Random change in allele frequency is genetic drift (caused BY CHANCE!) In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just BY CHANCE. Over time, a series of chance occurrences of this type can cause an allele to become common in a population = genetic drift

Genetic Drift (continued) May occur when a small group of individuals colonies a new habitat. These individuals may carry alleles in different relative frequencies than the larger population that they came from. If so, the population that they create will be genetically different from the parent population

RR RR Rr rr RR Rr RR Rr RR Rr Generation 1 p  0.7 q  0.3

Only 5 of 10 plants leave offspring p  0.7 q  0.3 p  0.5 q  0.5 RR RR rr RR Rr Rr rr RR RR rr Rr Rr RR Rr rr RR RR Rr Rr Rr Generation 1 p  0.7 q  0.3 p  0.5 q  0.5

Only 5 of 10 plants leave offspring Only 2 of 10 plants leave RR RR rr RR RR Rr Rr RR RR rr RR RR rr RR RR Rr Rr RR RR RR Rr rr RR RR RR Rr Rr Rr RR RR Generation 1 Generation 2 Generation 3 p  0.7 q  0.3 p  0.5 q  0.5 p  1.0 q  0.0

Genetic Drift: Bottleneck Effect The bottleneck effect is an example of genetic drift results from a drastic reduction in population size could be caused by earthquakes, floods, etc Passing through a “bottleneck,” a severe reduction in population size, decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and results in a loss of individual variation and hence adaptability.

Genetic Drift: Bottleneck Effect Original population Bottleneck event Surviving population

Genetic Drift: Bottleneck Effect Cheetahs appear to have experienced at least two genetic bottlenecks: during the last ice age, about 10,000 years ago, and during the 1800s, when farmers hunted the animals to near extinction. With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges.

Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Descendants Founding Population A Founding Population B

Gene Flow Gene flow is another source of evolutionary change, is separate from genetic drift, is genetic exchange with another population, may result in the gain or loss of alleles, and tends to reduce genetic differences between populations.

Hardy-Weinberg Equilibrium (page 259) Hardy and Weinberg – 2 scientists that asked: “Are there any conditions under which evolution will NOT occur?” If allele frequencies stay the same – the population does NOT evolve

Hardy-Weinberg Equilibrium (page 259) 5 conditions are required to maintain genetic equilibrium from generation to generation: Random mating – equal chance of passing genes Large population – genetic drift has less effect No movement into or out of the population – must maintain gene pool No mutation – can’t have new alleles No natural selection – no “survival of the fittest”

Hardy-Weinberg Equilibrium (page 259) Meeting these five conditions is difficult to do and often cannot be met, so evolution will occur Many organisms mate with chosen mates based on things like strength, color, etc Not all populations are large Migration occurs all the time Mutations are bound to happen Natural selection/survival of the fittest often cannot be avoided!

Natural Selection on Polygenic Traits Effects of natural selection on polygenic traits are very complex Can affect the distributions of phenotypes in any of three ways: Directional selection Stabilizing selection Disruptive selection

Directional Selection Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end Entire curve moves as the trait changes Ex. Finches and beak size – food became scarce; finches compete for food; bigger beaks are able to find food, survive and reproduce = beak size increases

Directional Selection Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce.

Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end of the curve Keeps the center of the curve at its current position, but narrows the graph Ex. Weight of human infants – small babies less likely to survive and large babies have difficulty being born = average-sized babies are favored

Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Disruptive Selection Individuals at upper and lower ends of the curve have higher fitness than individuals near the middle Can cause graph to split into two, creating two distinct phenotypes Ex. Bird – 2 different seed sizes (small and large); birds with large beaks and birds with small beaks survive = two distinct beak sizes

Disruptive Selection Disruptive Selection Key Largest and smallest seeds become more common. Key Low mortality, high fitness Population splits into two subgroups specializing in different seeds. Number of Birds in Population Number of Birds in Population High mortality, low fitness Beak Size Beak Size